[Qt-interest] QGraphicsScene and multitouch events

Sun Jan 16 16:52:24 CET 2011

Geovani,

FYI: I was not able to resolve my issue by your suggestions. I guess I will
have to step into Qt code to see what is happening.

Thanks,

Devendra Patel.

From: Luiz Geovani Vier [mailto:lgvier at gmail.com] 
Sent: Wednesday, January 12, 2011 6:59 PM
To: Devendra Patel
Cc: Sean Hayes; qt-interest at qt.nokia.com
Subject: Re: [Qt-interest] QGraphicsScene and multitouch events

Hello Devendra,

Since you are not using gestures, maybe you could try to unregister the
recognizers:

QGestureRecognizer::unregisterRecognizer(Qt::PanGesture);

QGestureRecognizer::unregisterRecognizer(Qt::SwipeGesture);

QGestureRecognizer::unregisterRecognizer(Qt::PinchGesture);

It seems the standard gesture recognizers are not able to dispatch the
events to multiple items on a scene simultaneously... Maybe that's
interfering with your touch events.

Also, try to accept the touch begin events

Good luck..

-Geovani

On Wed, Jan 12, 2011 at 5:38 AM, Devendra Patel <devpatel82 at gmail.com>
wrote:

Hi Sean,

I tried your suggestion but am facing some issues.

Firstly I am interested in raw touch events, not gestures. My goal is to
place two dials in QGraphicsView and rotate them simultaneously using
Multi-touch monitor. I am using Win 7 and Qt 4.7.

I wrote following code to see if I can rotate two dials simultaneously. I
can only rotate one dial simultaneously. I am able to rotate two dials
simultaneously if I place them on QWidget rather than
QGraphicsScene/QGraphicsView.

int main(int argc, char **argv)
{

     QApplication app(argc, argv);
     QCoreApplication::setAttribute(Qt::AA_DontCreateNativeWidgetSiblings);
// just make sure that native widgets are not created
     WCScene scene; //WCScene is derived from QGraphicsScene

     CKnob dial,dial1; //CKnob is derived from QDial
     dial.setGeometry(10,10,200,200);
     dial1.setGeometry(210,10,200,200);

     dial.setObjectName("1");
     dial1.setObjectName("2");
     dial.setAttribute(Qt::WA_AcceptTouchEvents);
     dial1.setAttribute(Qt::WA_AcceptTouchEvents);

     GraphicsView view(&scene); //GraphicsView derived from QGraphicsView
     scene.addWidget(&dial);
     scene.addWidget(&dial1);

     view.setAttribute((Qt::WA_AcceptTouchEvents));
     view.viewport()->setAttribute(Qt::WA_AcceptTouchEvents);
     view.show();
     return app.exec();
}

In all the custom classes I am handling events and print message. Below is
how I handle TouchBegin event in CKnob class.

 switch(e->type())
    {
    case QEvent::TouchBegin:
        qDebug("CKnob  QEvent::TouchBegin");
        return QDial::event(e);
    }

All the Mouse and Touch events are handled similarly in custom classes. In
GraphicsView I am overriding both viewportEvent(QEvent* ) and event(QEvent*)
to print messages.    

This is What I see when I try to rotate the dials.

GraphicsView viewportEvent::TouchBegin
SCene QEvent::TouchBegin
GraphicsView QEvent::TouchBegin
GraphicsView viewportEvent::MouseMove
GraphicsView viewportEvent::MouseButtonPress
CKnob  QEvent::MouseButtonPress
GraphicsView viewportEvent::MouseMove
CKnob  QEvent::MouseMove
GraphicsView viewportEvent::MouseMove
CKnob  QEvent::MouseMove
GraphicsView viewportEvent::MouseMove
CKnob  QEvent::MouseMove
GraphicsView viewportEvent::MouseMove
CKnob  QEvent::MouseMove
GraphicsView viewportEvent::MouseMove
CKnob  QEvent::MouseMove
GraphicsView viewportEvent::MouseMove
CKnob  QEvent::MouseMove
GraphicsView viewportEvent::MouseButtonRelease
CKnob  QEvent::MouseButtonRelease

I am puzzled to why I don't receive touchUpdate and touchEnd events at all,
neither in graphicsview, graphicsscene nor in CKnob.
Is there something wrong in my setup?

Thanks,
Devendra

2011/1/5 Sean Hayes <sean.t.hayes at vanderbilt.edu>

To get gestures to work in a QGraphicsView (in the scene) you have to do the
following:

*	In the QGraphicsView accept gestures on the viewport (e.g.,
viewport()->grabGesture(Qt::PinchGesture);)
*	In the QGraphicsObjects (and/or the QWidget) accept touch events
(i.e., setAcceptTouchEvents(true);), grab the gesture (e.g.,
grabGesture(Qt::PinchGesture);). 
*	Finally, in the event function (sceneEvent() for a QGraphicsObject
and event() for a QWidget) accept the touch begin events. An example below:

bool QMapTouchItem::sceneEvent(QEvent* pEvent) {

switch (pEvent->type()) {

case QEvent::TouchBegin: // Needed to get around Qt bug

pEvent->accept();

return true;

case QEvent::Gesture:

// handle the gesture

}

return QMapItem::sceneEvent(pEvent);

}

You can also check out this forum post.
http://www.qtcentre.org/threads/31582-Gestures-and-QGraphicsScene?p=172577.

On Tue, Jan 4, 2011 at 12:00 AM, Abhishek <abhishekworld at gmail.com> wrote:

I think you should look at QGraphicsProxyWdiget that makes more sense in
your case http://doc.qt.nokia.com/4.7-snapshot/qgraphicsproxywidget.html

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view(&scene);
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widget=new QWidget <http://doc.qt.nokia.com/latest/qwidget.html> ;
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48.     view.setDragMode(QGraphicsView
<http://doc.qt.nokia.com/latest/qgraphicsview.html> ::ScrollHandDrag);
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-- 
Abhishek Patil
http://thezeroth.net

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